Air electrode material powder for solid oxide fuel cell and its production process

ABSTRACT

To provide an air electrode material powder for a solid oxide fuel cell, comprising a novel LSCF powder having a highly uniform composition suitable as an air electrode material for a solid oxide fuel cell, and its production process. 
     A composite oxide having a perovskite structure and containing lanthanum, strontium, cobalt, iron and oxide, wherein the dispersion point determined by the peak intensity [La] of the Lα1 characteristic X-ray of lanthanum and the peak intensity [Sr] of the Lα1 characteristic X-ray of strontium as obtained by EPMA measurement, is present within a range of the formula (1) and the dispersion point determined by the peak intensity [Co] of the Kα1 characteristic X-ray of cobalt and the peak intensity [Fe] of the Kα1 characteristic X-ray of iron is present within a range of the formula (2): 
         a [La]−150≦[Sr]≦ a [La]+150   (1)
 
         b [Co]−300≦[Fe]≦ b [Co]+300   (2)
 
     wherein 0.2≦a≦1.0 and 0.1≦b≦4.0.

TECHNICAL FIELD

The present invention relates to an air electrode material powder for asolid oxide fuel cell, comprising a powder of a composite oxidecontaining lanthanum, strontium, cobalt, iron and oxygen and having aperovskite structure, particularly comprising a powder having a highuniformity of constituting elements in particles of the powder, and itsproduction process.

BACKGROUND ART

A solid oxide fuel cell is a fuel cell employing as an electrolyte asolid electrolyte having oxygen ion conductivity, and attracts attentionas a clean energy since the electrochemical reaction which causeselectromotive force is a hydrogen oxidation reaction, and no carbondioxide gas is formed. A solid oxide fuel cell usually has a stackstructure comprising single cells each comprising an air electrode as anoxide, a solid electrolyte and a fuel electrode connected by aninterconnector. Its operating temperature is usually about 1,000° C.,and decrease in the temperature is attempted and practically employed byvarious studies, however, it is at least about 600° C. and is still hightemperature.

Due to the cell structure and the high operating temperature of a solidoxide fuel cell, an air electrode material constituting the airelectrode is basically required to have such properties that (1) it hasa high oxygen ion conductivity, (2) it has a high electron conductivity,(3) its thermal expansion is similar to or about the same as that of anelectrolyte, (4) it has high chemical stability and has highcompatibility with other constituting materials, and (5) the sinteredproduct is required to be a porous product and it has a certainstrength, etc.

As a material of an air electrode which satisfies such properties, acomposite oxide represented by (La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃(hereinafter sometimes referred to as LSCF) having a perovskitestructure is energetically studied and developed as an air electrodematerial excellent in the electrode activity.

For example, Patent Document 1 discloses a ceramic powder containing asthe main component a lanthanum ferrite perovskite oxide. Specifically,it discloses a ceramic powder represented by a compositional formula(L_(1-x)AE_(x))_(1-y)(Fe_(z)M_(1-z))O_(3+δ), wherein L is one or more ofelements selected from the group consisting of rare earth elements suchas La, Sc and Y, AE is one or two of elements selected from the groupconsisting of Sr and Ca, M is one or more of elements selected from thegroup consisting of Co, Mg, Sc, Ti, V, Cr and Ni, 0<x<0.5, 0<y≦0.04 and0≦z<1 (claims).

And, specifically disclosed as a method for preparing the ceramic powderis to mix and pulverize lanthanum oxide, strontium carbonate, cobaltoxide and iron oxide in a solid phase using e.g. a mortar (hereinaftersometimes referred to as a solid phase method) and calcine the mixture(paragraphs [0032] and [0092] to [0094] (Example 1)).

However, by such a solid phase method, so long as four types of rawmaterial element-containing particles are pulverized and mixed in asolid phase, it is difficult in principle to obtain one having acompletely uniform composition at the micro level.

Further, Patent Document 1 discloses in Examples 2 to 3 and 6 to 11 anexample in which (La_(0.6)Sr0.4)_(1-z)(Co_(0.2)Fe_(0.8))O_(3+δ)(y=0,0.02, 0.04) having a specific surface area of 4 m²/g prepared by acitrate method, in addition to a solid phase method, is wet mixed withethanol and then pressure-molded. This method is the method disclosed inthe after-mentioned Comparative Example 1, and by this method, rawmaterial powders are mixed in a solution of citric acid alone, La₂O₃ asone of the raw material powders and a citrate after the reaction are notsufficiently dissolved, and the system is in a slurry state (hereinafterthis method will sometimes be referred to as a slurry method).

Further, Patent Document 2 discloses an air electrode material powderfor a solid electrolyte fuel cell, which is a perovskite composite oxidepowder represented by the formula ABO₃, wherein the A site comprises atleast one element selected from the group consisting of La and rareearth elements, and at least one element selected from the groupconsisting of Sr, Ca and Ba, and the B site comprises at least oneelement selected from the group consisting of Mn, Co, Fe, Ni and Cu,which is fine with an average particles size of at most 1 μm, and whichhas a narrow particle size distribution (claims).

Patent Document 2 relates to a LSCF powder having a small particle sizeand having a small dispersion of the particle size distribution, and toprepare such a powder, water-soluble nitrates of La, Sr, Co and Fe asraw material elements are dissolved in a predetermined proportion inwater, NH₄OH is added thereto to coprecipitate the respective insolublesalts, and the precipitates are dried and fired (hereinafter sometimesreferred to as a coprecipitation method) (paragraph [0032]).

Since in this coprecipitation method precipitates are formed from auniform solution, it is apparently considered that one having a uniformcomposition is easily formed, however, according to studies by thepresent inventors, in practice, the precipitates do not have a uniformcomposition since the pH at which insoluble salts of the respectiveelements precipitate and their crystal growth rates are different amongthe nitrates of the four types of elements. For example, a salt of oneelement is precipitated first and grows into large particles, and thenmicro crystals of the next element are precipitated on the largeparticles, and accordingly it is considered difficult in principle toobtain precipitates having a sufficiently uniform composition.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2009-35447

Patent Document 2: JP-A-2006-32132

DISCLOSURE OF INVENTION Technical Problem

As mentioned above, in conventional LSCF fine particles prepared by asolid phase method, a coprecipitation method or the like or further by aslurry method, the above four elements La, Sr, Co and Fe are hardlycompletely uniform in principle.

Under these circumstances, it is an object of the present invention isto provide a novel LSCF powder which is suitable as an air electrodematerial for a solid oxide fuel cell and which has a highly uniformcomposition, and a production process to obtain such a LSCF powderhaving a uniform composition.

Solution to Problem

The present inventors have conducted extensive studies to achieve theabove object and as a result, found that a novel particulate powderhaving a uniform composition even at the micro level, which has not beenpresent, can be obtained by reacting raw material powders containingfour elements of La, Sr, Co and Fe with an organic acid in a liquid toform a complex compound and substantially completely dissolving it, andspray drying the solution in a micro fine droplet state. The presentinvention has been accomplished on the basis of such a discovery.

That is, the present invention provides the following.

-   [1] An air electrode material powder for a solid oxide fuel cell,    comprising a powder of a composite oxide having a perovskite    structure and containing lanthanum, strontium, cobalt, iron and    oxygen, wherein the dispersion point determined by the peak    intensity [La] of the Lα1 characteristic X-ray of lanthanum and the    peak intensity [Sr] of the Lα1 characteristic X-ray of strontium,    obtained by EPMA (electron probe microanalyzer) measurement of    element distribution of the powder, is present within a range of the    formula (1), and the dispersion point determined by the peak    intensity [Co] of the Kα1 characteristic X-ray of cobalt and the    peak intensity [Fe] of the Kα1 characteristic X-ray of iron is    present within a range of the formula (2):

a[La]−150≦[Sr]≦a[La]+150   (1)

b[Co]−300≦[Fe]≦b[Co]+300   (2)

wherein 0.2≦a≦1.0 and 0.1≦b≦4.0.

-   [2] The air electrode material powder for a solid oxide fuel cell    according to the above [1], wherein the composition of the composite    oxide is represented by the formula (I):

(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃   (I)

wherein 0.1≦x≦0.5, 0.1≦y≦0.9, and 0.95≦a≦1.0.

-   [3] An air electrode for a solid oxide fuel cell, obtained by    sintering the air electrode material powder for a solid oxide fuel    cell as defined in the above [1] or [2].-   [4] A process for producing the air electrode material powder for a    solid oxide fuel cell as defined in the above [1] or [2], which    comprises forming compounds each containing a metal element    constituting the composite oxide into a solution using an aqueous    solution of an organic acid, spray drying the obtained solution and    firing the obtained dry powder.-   [5] The process for producing the air electrode material powder for    a solid oxide fuel cell according to the above [4], wherein the    number of moles of the organic acid used is from 2.3 to 10 times the    total number of moles of the metal elements of the compounds each    containing a metal element.-   [6] The process for producing the air electrode material powder for    a solid oxide fuel cell according to the above [4] or [5], wherein    the organic acid is at least one member selected from the group    consisting of maleic acid, lactic acid and malic acid.-   [7] The process for producing the air electrode material powder for    a solid oxide fuel cell according to the above [4] or [5], wherein    the organic acid is a mixture of citric acid with at least one    member selected from the group consisting of maleic acid, lactic    acid and malic acid.-   [8] The process for producing the air electrode material powder for    a solid oxide fuel cell according to the above [4] or [5], wherein    the organic acid is a mixture of citric acid with malic acid.-   [9] The process for producing the air electrode material powder for    a solid oxide fuel cell according to any one of the above [4] to    [8], wherein the organic acid is citric acid, and when the compounds    each containing a metal element constituting the composite oxide are    formed into a solution using the aqueous solution of the organic    acid, an ammonium compound is further added.-   [10] The process for producing the air electrode material powder for    a solid oxide fuel cell according to the above [9], wherein the    ammonium compound is at least one member selected from the group    consisting of ammonia, ammonium bicarbonate, ammonium carbonate and    an ammonium citrate.-   [11] The process for producing the air electrode material powder for    a solid oxide fuel cell according to any one of the above [4] to    [10], wherein each of the compounds each containing a metal element    constituting the composite oxide is at least one member selected    from the group consisting of a carbonate, an oxide, a hydroxide and    an organic acid salt of each metal element.-   [12] The process for producing the air electrode material powder for    a solid oxide fuel cell according to any one of the above [4] to    [11], wherein the firing temperature is from 700° C. to 1,300° C.

Advantageous Effects of Invention

According to the present invention, provided are an air electrodematerial powder for a solid oxide fuel cell, comprising a compositeoxide having a perovskite structure and containing lanthanum, strontium,cobalt, iron and oxygen and having a more highly uniform composition ascompared with one obtained by a conventional solid phase method,coprecipitation method or slurry method, and its production process.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium and the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum of LSCF fine particles in Example 1 in axes.

FIG. 2 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Fe] of the Kα1 characteristic X-ray of ironand the peak intensity [Co] of the Kα1 characteristic X-ray of cobalt ofLSCF fine particles in Example 1 in axes.

FIG. 3 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium and the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum of LSCF fine particles in Comparative Example 1 in axes.

FIG. 4 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Fe] of the Kα1 characteristic X-ray of ironand the peak intensity [Co] of the Kα1 characteristic X-ray of cobalt ofLSCF fine particles in Comparative Example 1 in axes.

FIG. 5 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium and the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum of LSCF fine particles in Comparative Example 3 in axes.

FIG. 6 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Fe] of the Kα1 characteristic X-ray of ironand the peak intensity [Co] of the Kα1 characteristic X-ray of cobalt ofLSCF fine particles in Comparative Example 3 in axes.

FIG. 7 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium and the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum of LSCF fine particles in Example 5 in axes.

FIG. 8 is a dispersion diagram illustrating a composition distributionshowing the peak intensity [Fe] of the Kα1 characteristic X-ray of ironand the peak intensity [Co] of the Kα1 characteristic X-ray of cobalt ofLSCF fine particles in Example 5 in axes.

DESCRIPTION OF EMBODIMENT

The air electrode material powder for a solid oxide fuel cell, which isa composite oxide having a perovskite structure and containinglanthanum, strontium, cobalt, iron and oxygen, of the present invention,preferably has a composition represented by the formula (I):

(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃   (I)

wherein 0.1≦x≦0.5, 0.1≦y≦0.9, and 0.95≦a≦1.0.

The composition of oxygen is stoichiometrically 3, but in some cases,the composition may be partially deficient in oxygen or oxygen may bepresent in excess, and so long as the composite oxide of the presentinvention contains perovskite structure(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃ as the main component, otherimpurity phases may be present.

In the above formula (I), when 0.1≦x≦0.5, 0.1≦y≦0.9, and 0.95≦a≦1.0 aresatisfied, the composite oxide keeps the perovskite structure, suchbeing favorable.

Examples of a composite oxide LSCF represented by the formula (I)include the following, but needless to say, it is not limited to suchexamples.

La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃

-   -   (LSCF: 6428, x=0.4, y=0.2, a=1.0)

LSCF: 6428 means that the molar ratio of La, Sr, Co and Fe in thecomposite oxide LSCF is 6:4:2:8, and the same applies hereinafter.

La_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃

-   -   (LSCF: 8228, x=0.2, y=0.2, a=1.0)

La_(0.6)Sr_(0.4)Co_(0.4)Fe_(0.6)O₃

-   -   (LSCF: 6446, x=0.4, y=0.4, a=1.0)

La_(0.64)Sr_(0.36)Co_(0.8)Fe_(0.82)O₃

-   -   (LSCF: 6428, x=0.36, y=0.18, a=1.0)

La_(0.588)Sr_(0.392)Co_(0.2)Fe_(0.8)O₃

-   -   (LSCF: 6428, x=0.4, y=0.2, a=0.98)

In the present invention, a LSCF powder (also referred to as fineparticles) of such a composite oxide is preferably obtained by themethod (referred to as a complete dissolution method) disclosed in thepresent invention, and the LSCF powder is characterized in that theuniformity of composition of the respective components (La, Sr, Co andFe) in the fine particles is very high as compared with known particles.

In the present invention, the uniformity (dispersion) of components ofthe composite oxide fine particles is evaluated by the peak intensitiesof the characteristics X-ray of lanthanum, strontium, cobalt and ironobtained by EPMA measurement, and specified as follows.

That is, by EPMA, of the composite oxide fine particles, peakintensities of the characteristic X-ray of lanthanum, strontium, cobaltand iron which are elements constituting the composite oxide aremeasured. On that occasion, of the powder of the composite oxide of thepresent invention, the dispersion point determined by the peak intensity[La] of the Lα1 characteristic X-ray of lanthanum and the peak intensity[Sr] of the Lα1 characteristic X-ray of strontium is present within arange of the formula (1), and the dispersion point determined by thepeak intensity [Co] of the Kα1 characteristic X-ray of cobalt and thepeak intensity [Fe] of the Kα1 characteristic X-ray of iron is presentwithin a range of the formula (2):

a[La]−150≦[Sr]≦a[La]+150   (1)

b[Co]−300≦[Fe]≦b[Co]+300   (2)

In the formula (1), the coefficient a of the peak intensity [La] of theLα1 characteristic X-ray of lanthanum is a value determined by the ratioof the concentration of strontium to the concentration of lanthanum inthe composite oxide fine particles, and is larger as the lanthanumconcentration is lower and the strontium concentration is higher.

Further, in the formula (2), the coefficient b of the peak intensity[Co] of the Kα1 characteristic X-ray of cobalt is a value determined bythe ratio of the concentration of iron to the concentration of cobalt inthe composite oxide fine particles, and is larger as the cobaltconcentration is lower and the iron concentration is higher.

In the present invention, it is more preferred that the dispersion pointdetermined by the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum and the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium is present within a range of the formula (1-2) and thedispersion point determined by the peak intensity [Co] of the Kα1characteristic X-ray of cobalt and the peak intensity [Fe] of the Kα1characteristic X-ray of iron is present within a range of the formula(2-2).

a[La]−125≦[Sr]≦a[La]+125   (1-2)

b[Co]−250≦[Fe]≦b[Co]+250   (2-2)

wherein a and b are as defined in the formulae (1) and (2).

This is because lanthanum, strontium, cobalt and iron as elementsconstituting the composite oxide are more uniformly distributed in theparticles.

The results of the EPMA measurement will be described in further detailwith reference to La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃(LSCF: 6428, x=0.4,y=0.2, a=1.0).

The above LSCF (6428) may be represented as ABO_(3,) that is,(La_(0.6)Sr_(0.4))(Co_(0.2)Fe_(0.8))O₃.

Originally, the ratio lanthanum/strontium of lanthanum to strontiumconstituting the A site should be constant at 0.6/0.4 i.e. 1.5 when thecomposition is completely uniform. Accordingly, in the EPMA measurement,the ratio of the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum to the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium should be constant.

Similarly, the ratio cobalt/iron of cobalt to iron constituting the Bsite should be constant at 0.2/0.8 i.e. 0.25 when the composition iscompletely uniform. Accordingly, in the EPMA measurement, the ratio ofthe peak intensity [Co] of the Kα1 characteristic X-ray of cobalt to thepeak intensity [Fe] of the Kα1 characteristic X-ray of iron should beconstant.

However, practically, the elements constituting the composite oxideparticles are not completely uniformly distributed but are distributedwith a non-uniformity at a certain extent i.e. with a dispersion ofconcentrations of the constituting elements.

With respect to the A site, there is a portion where the lanthanumconcentration is higher than the desired composition and the strontiumconcentration is lower than the desired composition, and there is aportion where the lanthanum concentration is lower than the desiredcomposition and the strontium concentration is higher than the desiredcomposition.

With respect to the B site, there is a portion where the cobaltconcentration is higher than the desired composition and the ironconcentration is lower than the desired composition, and there is aportion where the cobalt concentration is lower than the desiredcomposition and the iron concentration is higher than the desiredcomposition.

The composite oxide fine particles having a composition with such adispersion are subjected to EPMA measurement, and in a diagramillustrating a dispersion showing the peak intensity [Sr] of the Lα1characteristic X-ray of strontium in the vertical axis and the peakintensity [La] of the Lα1 characteristic X-ray of lanthanum in thehorizontal axis, [La] and [Sr] of the composite oxide particles having ahigh compositional uniformity of the present invention are present in aregion represented by the formula (1):

a[La]−150≦[Sr]≦a[La]+150   (1)

Here, the coefficient a of [La] is a value which greatly depends on thecomposition of the composite oxide and satisfies 0.2≦a≦1.0.

Further, in a diagram illustrating a dispersion showing the peakintensity [Fe] of the Kα1 characteristic X-ray of iron in the verticalaxis and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt in the horizontal axis, [Fe] and [Co] of the composite oxidehaving a high compositional uniformity of the present invention arepresent in a region represented by the formula (2):

b[Co]−300≦[Fe]≦b[Co]+300   (2)

Here, the coefficient b of [Co] is a value which greatly depends on thecomposition of the composite oxide and satisfies 0.3<b≦4.0.

Further, as shown in the after-mentioned Comparative Example, a LSCFpowder prepared by a conventional method has a greatly dispersedcomposition, and the degree of the dispersion evaluated by the peakintensities of the characteristic X-ray by the EPMA measurementindicates that the dispersion points determined by the peak intensitiesof the characteristic X-ray are present out of the ranges defined by thepresent invention.

Now, the process for producing the air electrode material for a solidoxide fuel cell having a composition represented by the formula (I)according to a preferred embodiment of the present invention will bedescribed.

(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃   (I)

(Preparation of Raw Material Powders)

As powders as raw materials of the air electrode material for a solidoxide fuel cell having a composition represented by the formula (I)(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃ according to a preferred embodimentof the present invention, commonly employed powders may suitably beused, and for example, an oxide, a hydroxide, a nitrate, a carbonate, anorganic acid salt and the like containing each of La, Sr, Co and Fe maybe mentioned.

Particularly in view of the environmental aspect and availability,preferred is a carbonate, a hydroxide or an oxide, and in view of highreactivity of the raw material, an organic acid salt such as a citrateis also preferred.

Further, as the raw material for one element, optional two or morecompounds selected from the group consisting of an oxide, a hydroxide, anitrate, a carbonate and an organic acid salt may be selected as theelement source.

The above raw material powders are weighed so that the respectiveelements La, Sr, Co and Fe achieve the desired composition representedby the formula (I).

The respectively weighed raw material powders are preferablypreliminarily pulverized to make particles fine, so that the dissolutionreaction quickly proceeds. Further, a part or all of the powders may bepreliminarily uniformly mixed. Mixing may be carried out by dry mixing,but preferably carried out by a wet mixing method, whereby a homogeneousraw material powder can be obtained in a relatively short time, andparticularly, pulverization may be carried out simultaneously withmixing.

An apparatus to carry out the wet mixing method is not particularlylimited, and is preferably one which can carry out pulverizationsimultaneously. For example, a ball mill, a bead mill, an attrition millor a colloid mill is preferred. Among them, one which employs a grindingmedium such as zirconia balls, for example a ball mill or a bead mill,is more preferably used. For example, the above grinding medium may beadded to the raw material powders, followed by pulverization and mixingusing a ball mill for from 12 to 24 hours. Pulverization and mixing by agrinding medium e.g. by a ball mill is preferred, whereby a strongershear force can be given, and a more homogeneous raw material mixedpowder is obtained.

(Organic Acid Aqueous Solution)

Separately, an aqueous solution of an organic acid is preliminarilyprepared. The organic acid is not particularly limited so long as itreacts with the compounds each containing a metal element to form acomplex and it can dissolve the complex, and is preferably at least onemember selected from the group consisting of maleic acid, formic acid,acetic acid, lactic acid and malic acid. Particularly, at least onemember selected from the group consisting of maleic acid, lactic acidand malic acid is preferably selected.

The concentration of the aqueous solution of the organic acid is notparticularly limited, and in view of the operation workability and witha view to sufficiently increasing the reaction rate, it is from 10 to 70wt %, preferably from 20 to 60 wt %, further preferably from 30 to 50 wt%.

Here, in a case where citric acid is used as the organic acid to formthe raw materials into a solution, it is preferred to use citric acidand at least one organic acid other than citric acid, selected from thegroup consisting of maleic acid, formic acid, acetic acid, lactic acidand malic acid in combination, and the organic acid other than citricacid is more preferably at least one member selected from the groupconsisting of maleic acid, lactic acid and malic acid. Among them, useof citric acid and malic acid in combination is particularly preferred.

In a case where citric acid and at least one organic acid other thancitric acid, selected from the group consisting of maleic acid, lacticacid and malic acid are used in combination, the molar ratio (the numberof moles of citric acid)/(the number of moles of at least one organicacid selected from the group consisting of maleic acid, lactic acid andmalic acid) of citric acid to at least one organic acid other thancitric acid, selected from the group consisting of maleic acid, lacticacid and malic acid, is preferably from 1/10 to 10/1, further preferablyfrom 1/3 to 3/1.

As the citric acid, any of anhydrous citric acid, citric acidmonohydrate and a mixture of anhydrous citric acid and citric acidmonohydrate may be used.

By use of citric acid and the organic acid other than citric acid incombination, the dissolution reaction (complex formation) will moreeasily proceed, and the effect is remarkable particularly when as theorganic acid other than citric acid, at least one member selected fromthe group consisting of maleic acid, lactic acid and malic acid is usedin combination. In the case of use of citric acid and malic acid incombination, in addition to the above effect to make the dissolutionreaction easily proceed, an effect to stabilize a solution in whichcompounds each containing a metal element constituting the compositeoxide are dissolved, will be obtained. That is, if an organic acid suchas malic acid is used alone, if the solution in which the raw materialsare dissolved is left to stand several days at room temperature, thecompounds each containing a metal element constituting the compositeoxide will re-precipitate, however, in the case of use of citric acidand malic acid in combination, the solution is stable as it is in asolution state.

In the present invention, when the compounds each containing a metalelement constituting the composite oxide are formed into a solution withan aqueous solution of citric acid, an ammonium compound may further beused simultaneously. Use of citric acid and an ammonium compound incombination is preferred, whereby the dissolution reaction (complexformation) will more easily proceed. The ammonium compound is preferablyat least one member selected from the group consisting of ammonia,ammonium bicarbonate, ammonium carbonate and an ammonium citrate. Theammonium citrate may, for example, be diammonium hydrogencitrate ortriammonium citrate. The amount of the ammonium compound is notparticularly limited so long as the raw material powders can bedissolved, and it is preferably from 1 to 10 times the number of molesof lanthanum ions, whereby the raw material powders will easily bedissolved.

The amount of use of the organic acid in the present invention ispreferably at least an amount with which the organic acid can form acomplex with the metal elements constituting the composite oxide and thecomplex can be sufficiently dissolved. Specifically, the amount of useof the organic acid is preferably from 2.3 to 10 times, particularlypreferably from 2.3 to 5 times the total number of moles of La, Sr, Coand Fe contained in La, Sr, Co and Fe sources which are compounds eachcontaining a metal element constituting the composite oxide. When theamount is at least 2.3 times, the metal elements will be substantiallycompletely dissolved, and when it is at most 10 times, the cost of theorganic acid can be reduced, and the amount of formation of carbondioxide gas tends to be small, and excessive decrease of the oxygenconcentration along with firing of the organic acid during firing can besuppressed.

(Dissolution Reaction)

The compounds each containing a raw material metal element constitutingthe composite oxide are dissolved in an aqueous solution of the aboveorganic acid to form a solution. An apparatus to carry out such adissolution reaction is not particularly limited, and for example, atank reactor equipped with a stirring means, a heating means, a rawmaterial powder supplying means and an organic acid aqueous solutionsupplying means, capable of making supplied raw material powders floatwithout precipitating and of allowing the raw material powders to reactwith the organic acid in a floating state, is preferred. As the stirringmeans, a conventional stirring machine, for example, any of a paddleagitator, a marine propeller stirrer and a turbine impeller mixer maysuitably be used. In the case of a small-scale reaction, a flask-shapedcontainer provided with a stirring machine may be used.

The method of contacting the powders of the metal element-containingcompounds with the organic acid aqueous solution is not particularlylimited so long as the reaction will efficiently be carried out andfinally a uniform solution can be obtained, since the dissolutionreaction is understood as a solid-liquid different phase reaction inview of chemical engineering. Usually, preferred is a method in whichthe organic acid aqueous solution is charged in a reactor, and the rawmaterial powders are added thereto with stirring, reacted and dissolved.

The raw material powders to be added may successively be added, or theraw material powders may preliminarily be mixed and the mixed powder issupplied all at once and reacted. Otherwise, such supply methods may becombined.

In a case where the raw material powders are successively added, first,a raw material powder containing one metal element e.g. a lanthanumoxide powder may be supplied to the organic acid aqueous solution,reacted and dissolved with heating, and then the other element compounds(e.g. strontium carbonate, cobalt carbonate, iron citrate and the like)are added and reacted all at once.

As the reaction temperature, the dissolution reaction is carried outpreferably under heating to a certain extent, whereby the dissolutionreaction will be promoted. The reaction temperature is usually from 30to 100° C., preferably from 40 to 90° C., further preferably from 50 to80° C. Further, the reaction time i.e. the time until which a uniformsolution is formed may vary depending upon the temperature, the organicacid concentration, the type of the organic acid and the raw materialelement-containing compounds, their particles sizes, etc., and isusually from 10 minutes to 10 hours, preferably from 30 minutes to 5hours, further preferably from about 1 to about 3 hours.

(Spray Drying, etc.)

In the present invention, the obtained solution is preferably dried by aspray dryer. By spray drying, the solution in which the respective rawmaterial metal elements are substantially completely dissolved in theorganic acid aqueous solution is supplied to a drying apparatus such asa flash dryer or a spray dryer and dried. The solution supplied to thedrying apparatus is formed into fine droplets in the apparatus, whichform a fluidized bed by heated air for drying, and the droplets aredried in an extremely short time while being transferred by the heatedair, whereby a dry powder is obtained in a short time. As a result,precipitation due to a difference in the solubility among La, Sr, Co andFe is suppressed, and the homogeneity of the respective metal elementsLa, Sr, Co and Fe in the dry powder will be high.

As an atomizer when a spray dryer is used, one having e.g. a rotatingdisk, a two-fluid nozzle or a pressure nozzle may properly be employed,and the temperature of the heated air for drying is preferably from 150to 300° C. at the inlet and from about 100 to about 150° C. at theoutlet.

By such spray drying, the solution in which all the raw material metalelements are dissolved to form a uniform phase is formed into finedroplets, and from the droplets moisture is evaporated and removedinstantaneously or in a very short time, whereby a dry powder (mixedpowder) having a solid phase with a uniform composition even at themicro level in principle precipitated is obtained.

The average particle size of the dry powder obtainable by spray dryingis preferably from 5 to 100 μm, more preferably from 20 to 70 μm.

(Firing)

Then, the spray dried mixed powder is preferably put in a firingcontainer and fired in a firing furnace. Firing basically preferablycomprises three steps differing in the firing temperature i.e. crudefiring, temporary firing and main firing, but may comprise two steps ofcrude firing and main firing, may comprises two steps of temporaryfiring and main firing, or may comprise one step of only main firing ofsequentially increasing the temperature. The material of the firingcontainer is not particularly limited, and for example, mullite orcordierite may be mentioned.

Of the firing furnace, a heat source may be an electric or gas shuttlekiln or in some cases, a roller hearth kiln or a rotary kiln and is notparticularly limited.

(Crude Firing)

In the crude firing step, an operation of increasing the temperature ofthe firing furnace to the desired firing temperature of preferably from300 to 600° C. at a temperature-raising rate of preferably from 20 to200° C./hour is carried out. By the temperature-raising rate being atleast 20° C./hour, the time until which the temperature reaches thedesired firing temperature will be shortened, thus improving theproductivity. Further, by the temperature-raising rate being at most200° C./hour, chemical change of the reacting substances at eachtemperature will sufficiently proceed.

The firing temperature in crude firing is preferably from 300 to 600°C., more preferably from 350 to 550° C. By the firing temperature beingat least 300° C., the carbon component is less likely to remain.Further, by the firing temperature being at most 600° C., an impurityphase is less likely to form in a product after main firing.

The firing time in crude firing is preferably from 4 to 24 hours, morepreferably from 8 to 20 hours. By the firing time being at least 4hours, the carbon component is less likely to remain. Further, if thefiring time exceeds 24 hours, although there is no change in theproduct, the productivity tends to decrease, and accordingly it ispreferably at most 24 hours. In crude firing, the temperature may bekept constant, for example at 400° C. for 8 hours, or may be increasedfor example from 300° C. to 400° C. at a rate of 20° C./hour.

The atmosphere in the firing furnace when crude firing is carried out isan oxygen-containing atmosphere, and is preferably the air atmosphere(in the air) or an atmosphere having an oxygen concentration of at most21 vol %. If the oxygen concentration exceeds 21 vol %, the carboncomponent in the raw material mixed powder will burn and the oxidationreaction partially proceeds and as a result, the constituting elementsin the product may unevenly be present in some cases, and accordingly anatmosphere having an oxygen concentration of at most 21 vol % ispreferred.

After crude firing is carried out, the temperature is decreased to roomtemperature. The temperature-decreasing rate is preferably from 40 to200° C./hour. By the temperature-decreasing rate being at least 40° C.,the productivity will improve. Further, by the temperature-decreasingrate being at most 200° C./hour, the firing container used is lesslikely to be broken by thermal shock. Here, after the crude firing step,the subsequent temporary firing step may be carried out withouttemperature decrease when the firing container is not changed and whencrushing is not carried out.

Then, the oxide obtained in the crude firing step is crushed as the caserequires. Crushing is usually carried out by dry crushing using apulverizer such as a cutter mill, a jet mill or an atomizer. The volumeaverage particle size after crushing is preferably from 1 to 50 μm, morepreferably from 1 to 20 μm.

(Temporary Firing)

Then, the crude-fired powder is subjected to temporary firing. In thetemporary firing step, the temperature of the firing furnace isincreased to the desired temporary firing temperature at atemperature-raising rate of from 50 to 800° C./hour, preferably from 50to 400° C./hour. By the temperature-raising rate being at least 50°C./hour, the time until which the temperature reaches the desired firingtemperature will be shortened, thus improving the productivity. Further,by the temperature-raising rate being at most 800° C./hour, the chemicalchange of the reacting substances at each temperature will sufficientlyproceed.

The temperature in temporary firing is preferably from 500 to 800° C.,more preferably from 500 to 700° C. By the temperature being at least500° C., the carbon component is less likely to remain. Further, by thetemperature being at most 800° C., the fired powder is less likely to beexcessively sintered.

The firing time is preferably from 4 to 24 hours, more preferably from 8to 20 hours. By the firing time being at least 4 hours, the carboncomponent is less likely to remain. Further, by the firing time being atmost 24 hours, the productivity will improve without any change in theproduct.

The atmosphere in the firing furnace when temporary firing is carriedout is preferably the same oxygen-containing atmosphere as theatmosphere at the time of crude firing. After temporary firing iscarried out for a predetermined time, the temperature is decreased toroom temperature. The temperature-decreasing rate is preferably from 40to 200° C./hour. By the temperature-decreasing rate being at least 40°C./hour, the productivity will not be lowered. Further, by thetemperature-decreasing rate being at most 200° C./hour, the firingcontainer is less likely to be broken by thermal shock.

Then, the oxide obtained by temporary firing is crushed as the caserequires in the same manner as after crude firing. Crushing is usuallycarried out by dry crushing using a pulverizer such as a cutter mill, ajet mill or an atomizer. The volume average particle size after crushingis preferably from 1 to 50 μm, more preferably from 1 to 20 μm.

(Main Firing)

Further, the temporary-fired powder is subjected to main firing. In themain firing step, the temperature in the firing furnace is increased tothe desired firing temperature at a temperature-raising rate of from 50to 800° C./hour, preferably from 50 to 400° C./hour. By thetemperature-raising rate being at least 50° C./hour, the time untilwhich the temperature reaches the desired firing temperature will beshortened, thus improving the productivity. Further, by thetemperature-raising rate being at most 800° C./hour, such will not occurthat the temperature reachs the desired firing temperature in such acondition that the chemical change of the reacting substances at eachtemperature does not sufficiently proceed and the reacting substancesare in a non-uniform state, and accordingly by-products will not form inthe fired product.

The temperature in main firing is preferably from 700 to 1,300° C., morepreferably from 750 to 1,250° C. By the temperature being at least 700°C. and at most 1,300° C., the desired crystal phase will effectivelyform.

The firing time is preferably from 4 to 24 hours, more preferably from 5to 20 hours. By the firing time being at least 4 hours, unreactedsubstances will not be mixed in the desired oxide, and a product in asingle crystalline phase will be obtained. Further, by the firing timebeing at most 24 hours, the productivity will not be lowered without anychange of the product.

The atmosphere in the firing furnace when main firing is carried out ispreferably the same oxygen-containing atmosphere as crude firing ortemporary firing. After main firing is carried out, the temperature isdecreased to room temperature. The temperature-decreasing rate ispreferably from 40 to 200° C./hour. By the temperature-decreasing ratebeing at least 40° C./hour, the productivity will not be lowered.Further, by the temperature-decreasing rate being at most 200° C./hour,the firing container used is less likely to be broken by thermal shock.

Then, the oxide obtained by main firing is crushed in the same manner asafter crude firing. Crushing is usually carried out by dry crushingusing a pulverizer such as a cutter mill, a jet mill or an atomizer. Thevolume average particle size of the powder after crushing is preferablyfrom 1 to 50 μm, more preferably from 1 to 20 μm. Then, as the caserequires, the powder may be pulverized by wet pulverization to adjustthe particle size. The above crude firing, temporary firing and mainfiring may be continuously carried out without decreasing thetemperature to room temperature after completion of each step or withoutcarrying out crushing after firing. That is, temporary firing may becarried out continuously after crude firing, main firing may be carriedout continuously after temporary firing, or three steps of crude firing,temporary firing and main firing may be carried out continuously.

(Molded Product, Sintered Product)

In the powder (fine particles) obtained by main firing, the respectivefine particles have a completely uniform composition(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃ (LSCF) even at the micro level, andby sintering a molded product of the powder, the molded sintered productmay suitably be used as the air electrode for a solid oxide fuel cell.That is, the molded sintered product takes over the highly uniformcomposition of the fine particles as it is, and accordingly a LSCFsintered product having an extremely uniform composition in principle isformed.

As a means to form a molded product and a sintered product, known meansare employed. For example, first, a (La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃powder is mixed with a binder, the mixture is filled in a mold having acertain volume, and a pressure is applied from above to prepare a moldedproduct. The method to apply a pressure may be mechanical uniaxial pressor cold isotactic press (CIP) or the like and is not particularlylimited.

Then, the molded product is subjected to heat treatment to obtain asintered product. The heat treatment temperature is preferably from1,000 to 1,450° C., more preferably from 1,000 to 1,300° C. By the heattreatment temperature being at least 1,000° C., mechanical strength ofthe molded product will sufficiently be maintained, and by the heattreatment temperature being at most 1,450° C., it is unlikely that apart of the formed LSCF is decomposed to form impurities, and that thehomogeneity of the elements constituting the lanthanum strontium cobaltiron composite oxide of the present invention will be given. The heattreatment time is preferably from 2 to 24 hours.

EXAMPLES

Now, the present invention will be described with reference tocomparison between Examples of the present invention (Examples 1 to 6)and Comparative Examples (Comparative Examples 1 to 3). However, it isunderstood that such Examples are merely examples of the embodiment ofthe present invention, and the present invention is by no meansrestricted to such specific Examples.

In the following, “%” means “mass (or weight) %” unless otherwisespecified.

Example 1 (1) (Preparation of Raw Material Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃.

That is, as shown in Table 1, 356.8 g of lanthanum oxide (La₂O₃) as a Lasource, 215.7 g of strontium carbonate (SrCO₃) as a Sr source, 92.16 gof cobalt carbonate (CoCO₃) manufactured by NIHON KAGAKU SANGYO CO.,LTD. having a Co content of 46.58% as a Co source and 876.7 g of ironcitrate (FeC₆H₅O₇.nH₂O) having a Fe content of 18.56% as a Fe sourcewere weighed (La:Sr:Co:Fe of 0.6:0.4:0.2:0.8 by the atomic ratio).

Separately, in a 20 L (liter) separable flask, 2,908 g of citric acidmonohydrate in an amount of three times the number of moles of La ions,twice respectively the numbers of moles of Sr ions and Co ions and equalto the number of moles of Fe ions since 1 mol of citric acid was alreadypresent in iron citrate, was added to 4.0 L of pure water of 55° C. toprepare a citric acid aqueous solution. 1,037 g (6 times the number ofmoles of La ions) of ammonium bicarbonate was added thereto, and thecompounds were dissolved in a stirring tank reactor at 28° C.

(2) (Intermediate Product and Drying)

Lanthanum oxide was charged to the citric acid aqueous solution to whichammonium bicarbonate was added, the mixture was heated to 70° C. andreacted at the temperature for 2 hours. Lanthanum oxide was completelydissolved, and a colorless and transparent solution was obtained.

Strontium carbonate, cobalt carbonate and iron citrate were added to thesolution, followed by reaction at 55° C. further for 2 hours. Therespective metal salts were completely dissolved, and a blackish browntransparent solution was obtained.

After completion of the reaction, the obtained solution was dried by aspray dryer to obtain a dry powder of a composite citrate as anintermediate product. As the spray dryer, BDP-10 Spray Bag Dryer(manufactured by OHKAWARA KAKOHKI CO., LTD.) was used, and drying wascarried out under conditions of an inlet temperature of 200° C., anoutlet temperature of 125° C. and an atomizer rotating speed of 15,000rpm.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was filled in four 30 cm square mullitecrucibles and fired in the air in an electric furnace at 400° C. for 10hours to decompose the organic substances (crude firing). Thetemperature-raising rate from room temperature to 400° C. was 400° C./3hours, and the temperature-decreasing rate from 400° C. to roomtemperature was 400° C./4 hours.

The obtained crude-fired powder was filled in one 30 cm square mullitecrucible and fired in the air in an electric furnace at 600° C. for 10hours to decompose remaining carbon (temporary firing). Thetemperature-raising rate from room temperature to 500° C. was 500° C./3hours, the temperature-raising rate to 600° C. was 100° C./2 hours, andthe temperature-decreasing rate from 600° C. to room temperature was600° C./6 hours, and a temporary-fired powder was obtained.

The temporary-fired powder was filled in one 30 cm square mullitecrucible and fired in the air in an electric furnace at 1,000° C. for 6hours to obtain the desired LSCF final powder(La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃) (main firing). Thetemperature-raising rate from room temperature to 700° C. was 700° C./4hours, the temperature-raising rate to 1,000° C. was 100° C./1 hour, andthe temperature-raising rate from 1,000° C. to room temperature was 100°C./1 hour. After main firing, the fired product was crushed to obtain aLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ powder.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ final powderwas collected and subjected to powder X-ray diffraction measurementusing CuKα as an X-ray source to identify the crystal phase. For theX-ray diffraction measurement, RINT2200V manufactured by RigakuCorporation was used. As a result, the powder was confirmed to have aperovskite structure having rhombohedral crystals (113).

(ii) EPMA Analysis

Further, the above powder was subjected to EPMA analysis, and the peakintensities of the Lα1 characteristic X-ray of lanthanum, the Lα1characteristic X-ray of strontium, the Kα1 characteristic X-ray of ironand the Kα1 characteristic X-ray of cobalt were measured by the unit ofcount. A measurement sample was prepared by embedding the powder in anepoxy resin and mechanically polished to prepare a cross section, andcoating the cross section with carbon in a thickness of 30 nm.

For measurement, FE-EPMA JXA-8500F manufactured by JEOL Ltd. was used,electron beam applied to the measurement sample was accelerated at 15kV, and measurement was carried out under conditions of an irradiationcurrent of 30 nA and a probe diameter of 1 μm. The characteristic X-raywas measured in a range (250×250 μm) of 500×500 point each point being a0.5 μm square, and in each point, the measurement time for the specificX-ray of each element was 50 milliseconds. Further, the cumulated numberwas one.

Further, for spectroscopy of the Lα1 characteristic X-ray of lanthanumand the Lα1 characteristic X-ray of strontium, PETJ and TAP wererespectively used as the analyzing crystals, and for spectroscopy of theKα1 characteristic X-ray of iron and the Kα1 characteristic X-ray ofcobalt, LIFH was used.

FIG. 1 is a diagram showing the peak intensity [Sr] of the Lα1characteristic X-ray of strontium as obtained by the EPMA measurement ofthe above powder in the vertical axis and the peak intensity [La] of theLα1 characteristic X-ray of lanthanum in the horizontal axis. Thedispersion point showing the composition distribution, determined by thepeak intensity [Sr] of the Lα1 characteristic X-ray of strontium and thepeak intensity [La] of the Lα1 characteristic X-ray of lanthanum, isfound to be distributed in a region represented by the formula (3) inthe vicinity of [Sr]=0.8200[La]+4.3384:

0.82[La]−88≦[Sr]≦0.82[La]+88   (3)

Further, FIG. 2 is a diagram showing the peak intensity [Fe] of the Kα1characteristic X-ray of iron as obtained by the EPMA measurement of thepowder in the vertical axis and the peak intensity [Co] of the Kα1characteristic X-ray of cobalt in the horizontal axis. The dispersionpoint showing the composition distribution, determined by the peakintensity [Fe] of the Kα1 characteristic X-ray of iron and the peakintensity [Co] of the Kα1 characteristic X-ray of cobalt is found to bedistributed in a region represented by the formula (4) in the vicinityof [Fe]=3.4104[Co]+1.7013:

3.4[Co]−133≦[Fe]≦3.4[Co]+133   (4)

(iii) Particle Size Distribution Measurement

A small amount of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was dispersed indeionized water as follows to prepare a sample. Using as a dispersingagent an aqueous solution having a concentration of 0.24 wt % usingsodium diphosphate decahydrate manufactured by Wako Pure ChemicalIndustries, Ltd., 10 ml of a dispersion was prepared from about 0.001 gof La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ and the dispersing agent, and thedispersion was irradiated with ultrasonic waves for 3 minutes to preparea sample. Using the sample, the particle size distribution ofLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was measured by a laserdiffraction/scattering type particle size distribution analyzer LA-920manufactured by HORIBA Ltd. Immediately before the measurement, anultrasonic treatment at an output of 30 W was carried out for 180seconds. As a result, the volume average particle size D₅₀ was 15.1 μm.

Example 2 (1) (Preparation of Raw Material Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ in the same manner as in Example 1(see Table 1).

Separately, in a 20 L separable flask, 1,837 g of citric acidmonohydrate, 1,954 g of malic acid and 4.0 L of pure water were added toprepare an aqueous solution comprising a mixture of citric acid andmalic acid, which was dissolved at 28° C. in a stirring tank reactorwithout adding an ammonium source such as ammonium bicarbonate.

(2) (Intermediate Product and Drying)

Lanthanum oxide was charged to the above aqueous solution of a mixtureof citric acid and malic acid, and the mixture was heated to 55° C.,followed by reaction at the temperature for 2 hours. Lanthanum oxide wascompletely dissolved, and a colorless transparent solution was obtained.

To the solution, strontium carbonate, cobalt carbonate and iron citratewere added, followed by reaction at 55° C. further for 2 hours. Themetal salts were completely dissolved, and a blackish brown transparentsolution was obtained.

After completion of the reaction, the obtained solution was dried by aspray dryer to obtain a dry powder of a composite organic acid salt asan intermediate product. As the spray dryer, the same BDP-10 Spray BagDryer (manufactured by OHKAWARA KAKOHKI CO., LTD.) as in Example 1 wasused, and drying was carried out under the same conditions as in Example1.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was filled in four 30 cm square mullitecrucibles, and subjected to crude firing in the air in an electricfurnace in the same manner as in Example 1 and then to temporary firingand further to main firing.

After main firing, the fired powder was crushed to obtain aLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ powder.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurement inthe same manner as in Example 1. As a result, the powder was confirmedto have a perovskite structure having rhombohedral crystals (113).

(ii) EPMA Analysis

Further, the powder was subjected to analysis by EPMA in the same manneras in Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum ofthe powder was distributed in a region represented by the formula (5) inthe vicinity of [Sr]=0.8581[La]+3.8959:

0.86[La]−108≦[Sr]≦0.86[La]+108   (5)

Further, the distribution point showing the composition distribution,determined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt of the powder was distributed in a region represented by theformula (6) in the vicinity of [Fe]=3.2425[Co]+3.3556:

3.2[Co]−186≦[Fe]≦3.2[Co]+186   (6)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was 15.7 μm.

Comparative Example 1 (1) (Intermediate Product Obtained by Adding RawMaterial Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ in the same manner as in Example 1(see Table 1).

Separately, 2 L (liter) of pure water was added into a 20 L separableflask, lanthanum oxide was added and the liquid temperature wasmaintained at 50° C., and hydration reaction (La₂O₃+3H₂O→2La(OH)₃) wascarried out for 2 hours. To the obtained mixture, strontium carbonate,cobalt carbonate and iron citrate were added and dispersed for one hour.816 g of citric acid monohydrate was further added, followed by reactionfor 2 hours to obtain a brown slurry.

The amount of the citric acid monohydrate required is equal respectivelyto the numbers of moles of La ions and Co ions and 2/3 time the numberof moles of Sr ions.

(2) (Drying of Intermediate Product)

The prepared slurry obtained by adding citric acid monohydrate was putin a stainless steel vat, and dried one day in a shelf dryer set at 110°C.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was filled in four 30 cm square mullitecrucibles, and subjected to crude firing in the air in an electricfurnace in the same manner as in Example 1 and then to temporary firingand further to main firing at 600° C. for 6 hours.

The temperature program of main firing was such that thetemperature-raising rate from room temperature to 600° C. was 600° C./4hours, and the temperature-decreasing rate from 600° C. to roomtemperature was 100° C./1 hour. After main firing, the fired powder wascrushed to obtain a La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ powder.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurement inthe same manner as in Example 1. As a result, the powder comprised 77.9wt % of rhombohedral crystals (113), 19.3 wt % of SrCO₃ and 2.8 wt % ofLa₂O₂CO₃ and contained a large quantity of impurity phases.

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

FIG. 3 is a diagram showing the peak intensity [Sr] of the Lα1characteristic X-ray of strontium as obtained by EPMA measurement of thepowder in the vertical axis and the peak intensity [La] of the Lα1characteristic X-ray of lanthanum in the horizontal axis. The dispersionpoint showing the composition distribution, determined by the peakintensity [Sr] of the Lα1 characteristic X-ray of strontium and the peakintensity [La] of the Lα1 characteristic X-ray of lanthanum is found tobe distributed in a region represented by the formula (7) in thevicinity of [Sr]=0.7483[La]+16.8516:

0.75[La]−332≦[Sr]≦0.75[La]+332   (7)

Further, the FIG. 4 is a diagram showing the peak intensity [Fe] of theKα1 characteristic X-ray of iron as obtained by the EPMA measurement ofthe powder in the vertical axis and the peak intensity [Co] of the Kα1characteristic X-ray of cobalt in the horizontal axis. The dispersionpoint showing the composition distribution, determined by the peakintensity [Fe] of the Kα1 characteristic X-ray of iron and the peakintensity [Co] of the Kα1 characteristic X-ray of cobalt, is found to bedistributed in a region represented by the formula (8) in the vicinityof [Fe]=3.1284[Co]+12.3446:

3.1[Co]−250≦[Fe]≦3.1 [Co]+250   (8)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was 15.3 μm.

Comparative Example 2

-   (1) The same operation as in Comparative Example 1 was carried out    except that main firing was carried out at 1,200° C. for 6 hours, to    obtain a final powder represented as La0.6Sr_(0.4)Co_(0.2)Fe_(0.8)O₃    (see Table 1).

The temperature program of main firing was such that thetemperature-raising rate from room temperature to 700° C. was 700° C./4hours, the temperature-raising rate to 1,000° C. was 100° C./1 hour, thetemperature-raising rate to 1,200° C. was 200° C./3 hours, and thetemperature-decreasing rate from 1,200° C. to room temperature was 100°C./1 hour.

(2) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurement inthe same manner as in Example 1. As a result, the powder was confirmedto have a perovskite structure having rhombohedral crystals (113).

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum ofthe powder is distributed in a region represented by the formula (9) inthe vicinity of [Sr]=0.7546[La]+5.8214:

0.75[La]−157≦[Sr]≦0.75[La]+157   (9)

Further, the dispersion point showing the composition distribution,determined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt of the powder was distributed in a region represented by theformula (10) in the vicinity of [Fe]=3.1912[Co]+9.8977:

3.2[Co]−1309≦[Fe]≦3.2[Co]+1309   (10)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particles sizeD₅₀ of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was 15.5 μm.

Comparative Example 3 (1) Weighing and Mixing of Raw Material Powders

Raw material powders were weighed to formLa_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ in the same manner as in Example 1except that 234.0 g of iron oxide (Fe₂O₃) was used instead of ironcitrate as the Fe source (see Table 1).

The weighed raw material powders were put in a nylon pot having aninternal capacity of 5 L, mixed in a ball mill for 24 hours togetherwith 10 kg of zirconia balls of 5 mm in diameter and 2.2 L of deionizedwater and dried in a dryer set at 110° C. for 24 hours to obtain a rawmaterial mixed powder.

(2) (Crude Firing, Temporary Firing and Main Firing)

The obtained raw material mixed powder was filled in four 30 cm squaremullite crucibles, and subjected to crude firing in the air in anelectric furnace in the same manner as in Example 1 and then totemporary firing and further to main firing at 1,100° C. for 6 hours.

The temperature program of main firing was such that thetemperature-raising rate from room temperature to 1,100° C. was 1,100°C./6 hours, and the temperature-decreasing rate from 1,100° C. to roomtemperature was 100° C./1 hour. After main firing, the fired powder wascrushed to obtain a La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ powder.

(3) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurement inthe same manner as in Example 1. As a result, the powder was confirmedto have a perovskite structure having rhombohedral crystals (113).

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

FIG. 5 is a diagram showing the peak intensity [Sr] of the Lα1characteristic X-ray of strontium as obtained by EPMA measurement of thepowder in the vertical axis and the peak intensity [La] of the Lα1characteristic X-ray of lanthanum in the horizontal axis.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum isfound to be distributed in a region represented by the formula (11) inthe vicinity of [Sr]=0.7883[La]+4.6565:

0.79[La]−146≦[Sr]≦0.79[La]+146   (11)

Further, the FIG. 6 is a diagram showing the peak intensity [Fe] of theKα1 characteristic X-ray of iron as obtained by the EPMA measurement ofthe powder in the vertical axis and the peak intensity [Co] of the Kα1characteristic X-ray of cobalt in the horizontal axis.

The dispersion point showing the composition distribution, determined bythe peak intensity [Fe] of the Kα1 characteristic X-ray of iron and thepeak intensity [Co] of the Kα1 characteristic X-ray of cobalt in FIG. 6,is found to be distributed in a region represented by the formula (12)in the vicinity of [Fe]=3.0452[Co]+13.1548:

3.1[Co]−1283≦[Fe]≦3.1[Co]+1283   (12)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ was 14.8 μm.

Example 3 (1) (Preparation of Raw Material Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃. That is, 847.1 g of lanthanumcarbonate (L₂(CO₃)₃.8H₂O) as a La source, 102.3 g of strontium carbonate(SrCO₃) as a Sr source, 65.17 g of cobalt hydroxide (Co(OH)₂) as a Cosource and 338.7 g of hydrated iron hydroxide (Fe(OH)₃) having a Fecontent of 45.58% as a Fe source (La:Sr:Co:Fe of 0.8:0.2:0.2:0.8 by theatomic ratio) were weighed. Separately, into a 20 L separable flask,1,910 g of maleic acid and 8.0 L (liter) of pure water were added toprepare a maleic acid aqueous solution, which was dissolved at 55° C.with stirring without adding an ammonium source such as ammoniumbicarbonate.

(2) (Intermediate Product and Drying)

To the above maleic acid aqueous solution, lanthanum carbonate,strontium carbonate, cobalt hydroxide and iron hydroxide were added andheated to 70° C., followed by reaction at the temperature for 2 hours.The metal salts were completely dissolved, and a blackish browntransparent solution was obtained.

After completion of the reaction, the obtained solution was dried in aspray dryer to obtain a dry powder of a composite maleate as anintermediate product. As the spray dryer, the same BDP-10 Spray BagDryer (manufactured by OHKAWARA KAKOHKI CO., LTD.) as in Example 1 wasused, and drying was carried out under the same conditions as in Example1.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was subjected to crude firing and temporaryfiring under the same conditions as in Example 1.

The temporary-fired powder was filled in one 30 cm square mullitecrucible, and fired in the air in an electric furnace at 1,200° C. for 6hours and crushed to obtain a LSCF final powder(La_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃) (main firing). Thetemperature-raising rate from room temperature to 700° C. was 700° C./4hours, the temperature-raising rate to 1,200° C. was 100° C./1 hour, andthe temperature-decreasing rate from 1,200° C. to room temperature was100° C./1 hour.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurementusing CuKα as an X-ray source to identify the crystal phase. For X-raydiffraction measurement, the same RINT2200V manufactured by RigakuCorporation as in Example 1 was used. As a result, the powder wasconfirmed to have a perovskite structure having rhombohedral crystals(113).

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum ofthe powder was distributed in a region represented by the formula (13)in the vicinity of [Sr]=0.3135[La]+4.6452:

0.31[La]−115≦[Sr]≦0.31[La]+115   (13)

Further, the dispersion point showing the composition distribution,determined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt of the powder was distributed in a region represented by theformula (14) in the vicinity of [Fe]=2.4330[Co]+17.7364:

2.4[Co]−267≦[Fe]≦2.4[Co]+267   (14)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃ was 15.9 μm.

Example 4 (1) (Preparation of Raw Material Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.8)Sr_(0.2)Co_(0.8)Fe_(0.2)O₃.

That is, 521.5 g of lanthanum hydroxide (La(OH)₃) as a La source, 101.5g of strontium carbonate (SrCO₃) as a Sr source, 347.0 g of cobaltcarbonate (CoCO₃) manufactured by NIHON KAGAKU SANGYO CO., LTD. having aCo content of 46.58% as a Co source and 84.00 g of hydrated ironhydroxide (Fe(OH)₃) having a Fe content of 45.58% as a Fe source(La:Sr:Co:Fe of 0.8:0.2:0.8:0.2 by the atomic ratio) were weighed.Separately, into a 20 L separable flask, 2,985 g of lactic acid having apurity of 90% and 4.0 L (liter) of pure water were added to prepare alactic acid aqueous solution, which was dissolved at 50° C. withstirring without adding an ammonium source such as ammonium bicarbonate.

(2) (Intermediate Product and Drying)

To the above lactic acid aqueous solution, lanthanum carbonate,strontium carbonate, cobalt carbonate and hydrated iron hydroxide wereadded, followed by reaction at 50° C. for 2 hours. The metal salts werecompletely dissolved, and a blackish brown transparent solution wasobtained.

After completion of the reaction, the obtained solution was dried in aspray dryer to obtain a dry powder of a composite lactate as anintermediate product. As the spray dryer, the same BDP-10 Spray BagDryer (manufactured by OHKAWARA KAKOHKI CO., LTD.) as in Example 1 wasused, and drying was carried out under the same conditions as in Example1.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was subjected to crude firing and temporaryfiring under the same conditions as in Example 1.

The temporary-fired powder was filled in one 30 cm square mullitecrucible, and fired in the air in an electric furnace at 750° C. for 6hours to obtain the desired LSCF final powder(La_(0.8)Sr_(0.2)Co_(0.8)Fe_(0.2)O₃) (main firing).

The temperature-raising rate from room temperature to 750° C. was 750°C./4 hours, and the temperature-decreasing rate from 750° C. to roomtemperature was 100° C./1 hour.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.8)Sr_(0.2)Co_(0.8)Fe_(0.2)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurementusing CuKα as an X-ray source to identify the crystal phase. For X-raydiffraction measurement, the same RINT2200V manufactured by RigakuCorporation as in Example 1 was used. As a result, the powder wasconfirmed to have a perovskite structure having rhombohedral crystals(113).

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum ofthe powder was distributed in a region represented by the formula (15)in the vicinity of [Sr]=0.3146[La]+3.8285:

0.31[La]−92≦[Sr]≦0.31[La]+92   (15)

Further, the dispersion point showing the composition distribution,determined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt of the powder was distributed in a region represented by theformula (16) in the vicinity of [Fe]=0.2271[Co]+2.7800:

0.23[Co]−68≦[Fe]≦0.23[Co]+68   (16)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.8)Sr_(0.2)Co_(0.8)Fe_(0.2)O₃ was 16.1 μm.

Example 5 (1) (Preparation of Raw Material Powders and Organic Acid)

Raw material powders were weighed to formLa_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O₃.

That is, 412.1 g of lanthanum hydroxide (La(OH)₃) as a La source, 213.9g of strontium carbonate (SrCO₃) as a Sr source, 365.6 g of cobaltcarbonate (CoCO₃) having a Co content of 46.58% manufactured by NIHONKAGAKU SANGYO CO., LTD. as a Co source and 88.50 g of hydrated ironhydroxide (Fe(OH)₃)₃ having a Fe content of 45.58% as a Fe source(La:Sr:Co:Fe of 0.6:0.4:0.8:0.2 by the atomic ratio) were weighed asshown in Table 1. Separately, into a 20 L separable flask, 1,973 g ofcitric acid monohydrate and 4.0 L (liter) of pure water at 55° C. wereadded to prepare a citric acid aqueous solution, and 1,470 g ofdiammonium hydrogencitrate was added thereto and dissolved in a stirringtank reactor at 28° C.

(2) (Intermediate Product and Drying)

To the above citric acid aqueous solution into which disodiumhydrogencitrate was added, lanthanum hydroxide was charged and heated at70° C., followed by reaction at the temperature for 2 hours. Lanthanumhydroxide was completely dissolved, and a colorless transparent solutionwas obtained. To the solution, strontium carbonate, cobalt carbonate andhydrated iron hydroxide were added, followed by reaction at 55° C.further for 2 hours. The metal salts were completely dissolved, and ablackish brown transparent solution was obtained.

After completion of the reaction, the obtained solution was dried by aspray dryer to obtain a dry powder of a composite citrate as anintermediate product. As the spray dryer, the same BDP-10 Spray BagDryer (manufactured by OHKAWARA KAKOHKI CO., LTD.) as in Example 1 wasused, and drying was carried out under the same conditions as in Example1.

(3) (Crude Firing, Temporary Firing and Main Firing)

The obtained dry powder was subjected to crude firing and temporaryfiring under the same conditions as in Example 1.

The temporary-fired powder was filled in one 30 cm square mullitecrucible, and fired in the air in an electric furnace at 800° C. for 6hours and crushed to obtain a LSCF final powder(La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O₃) (main firing). Thetemperature-raising rate from room temperature to 800° C. was 800° C./4hours, and the temperature-decreasing rate from 800° C. to roomtemperature was 100° C./1 hour.

(4) (Component Analysis) (i) XRD Analysis

A small amount of the La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O₃ final powderwas collected, and subjected to powder X-ray diffraction measurementusing CuKα as an X-ray source to identify the crystal phase. For X-raydiffraction measurement, the same RINT2200V manufactured by RigakuCorporation as in Example 1 was used. As a result, the powder wasconfirmed to have a perovskite structure having rhombohedral crystals(113).

(ii) EPMA Analysis

Further, the powder was subjected to EPMA analysis in the same manner asin Example 1, and peak intensities of the Lα1 characteristic X-ray oflanthanum, the Lα1 characteristic X-ray of strontium, the Kα1characteristic X-ray of iron and the Kα1 characteristic X-ray of cobaltwere measured by the unit of count.

FIG. 7 is a diagram showing the peak intensity [Sr] of the Lα1characteristic X-ray of strontium as obtained by the EPMA measurement ofthe powder in the vertical axis and the peak intensity [La] of the Lα1characteristic X-ray of lanthanum in the horizontal axis.

In FIG. 7, the dispersion point showing the composition distribution,determined by the peak intensity [Sr] of the Lα1 characteristic X-ray ofstrontium and the peak intensity [La] of the Lα1 characteristic X-ray oflanthanum was distributed in a region represented by the formula (17) inthe vicinity of [Sr]=0.9095[La]+3.9086:

0.91[La]−110≦[Sr]≦0.91[La]+110   (17)

Further, FIG. 8 is a diagram showing the peak intensity [Fe] of the Kα1characteristic X-ray of iron as obtained by the EPMA measurement of thepowder in the vertical axis and the peak intensity [Co] of the Kα1characteristic X-ray of cobalt in the horizontal axis.

In FIG. 8, the dispersion point showing the composition distributiondetermined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt was distributed in a region represented by the formula (18) inthe vicinity of [Fe]=0.1722[Co]+5.2756:

0.17[Co]−128≦[Fe]≦0.17[Co]+128   (18)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O₃ was 15.8 μm.

Example 6 (1) (Preparation of Final Powder)

Raw material powders were weighed to form(La_(0.6)Sr_(0.4))_(0.99)Co_(0.2)Fe_(0.8)O₃.

That is, as shown in Table 1, 355.5 g of lanthanum oxide (La₂O₃) as a Lasource, 214.9 g of strontium carbonate (SrCO₃) as a Sr source, 92.75 gof cobalt carbonate (CoCO₃) manufactured by NIHON KAGAKU SANGYO CO.,LTD. having a Co content of 46.58% as a Co source and 882.3 g of ironcitrate (FeC₆H₅O₇.nH₂O) having a Fe content of 18.56% as a Fe sourcewere weighed (La:Sr:Co:Fe of 0.594:0.396:0.2:0.8 by the atomic ratio).

Separately, in a 20 L (liter) separable flask, 2,907 g of citric acidmonohydrate in an amount of three times the number of moles of La ions,twice respectively the numbers of moles of Sr ions and Co ions and equalto the number of moles of Fe ions, was added to 4.0 L (liter) of purewater of 55° C. to prepare a citric acid aqueous solution. 1,033 g (6times to the metal La) of ammonium bicarbonate was added thereto, andthe compounds were dissolved at 28° C.

Thereafter, the same operation as in Example 1 was carried out exceptthat the firing temperature in main firing was 800° C., to obtain a(La_(0.6)Sr_(0.4))_(0.99)Co_(0.2)Fe_(0.8)O₃ powder.

The temperature-raising rate from room temperature to 800° C. was 800°C./4 hours, and the temperature-decreasing rate from 800° C. to roomtemperature was 100° C./1 hour.

(2) (Component Analysis) (i) XRD Analysis

A small amount of the (La_(0.6)Sr_(0.4))_(0.99)Co_(0.2)Fe_(0.8)O₃ finalpowder was collected, and subjected to powder X-ray diffractionmeasurement using CuKα as an X-ray source to identify the crystal phase.For the X-ray diffraction measurement, RINT2200V manufactured by RigakuCorporation was used. As a result, the powder was confirmed to have aperovskite structure having rhombohedral crystals (113).

(ii) EPMA Analysis

Further, the above powder was subjected to EPMA analysis in the samemanner as in Example 1, and the peak intensities of the Lα1characteristic X-ray of lanthanum, the Lα1 characteristic X-ray ofstrontium, the Kα1 characteristic X-ray of iron and the Kα1characteristic X-ray of cobalt were measured by the unit of count.

The dispersion point showing the composition distribution, determined bythe peak intensity [Sr] of the Lα1 characteristic X-ray of strontium andthe peak intensity [La] of the Lα1 characteristic X-ray of lanthanum,was distributed in a region represented by the formula (19) in thevicinity of [Sr]=0.7627[La]+3.8587:

0.76[La]−118≦[Sr]≦0.76[La]+118   (19)

Further, the dispersion point showing the composition distribution,determined by the peak intensity [Fe] of the Kα1 characteristic X-ray ofiron and the peak intensity [Co] of the Kα1 characteristic X-ray ofcobalt was distributed in a region represented by the formula (20) inthe vicinity of [Fe]=3.2986[Co]+8.2651:

3.3[Co]−228≦[Fe]≦3.3[Co]+228   (20)

(iii) Particle Size Distribution Measurement

Particle size distribution measurement was carried out in the samemanner as in Example 1. As a result, the volume average particle sizeD₅₀ of (La_(0.6)Sr₀₄)_(0.99)Co_(0.2)Fe_(0.8)O₃ was 15.1 μm.

The above results are shown in Table 1.

TABLE 1 Metal Amount Amount Amount Main firing Raw material content usedused Ammonium used temperature Composition compound (%) (g) Organic acid(g) compound (g) (° C.) Ex. 1 La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃Lanthanum oxide 85.08 356.8 Citric acid 2,908 Ammonium 1,037 1,000Strontium carbonate 59.18 215.7 monohydrate bicarbonate Cobalt carbonate46.58 92.16 Iron citrate 18.56 876.7 Ex. 2La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ Lanthanum oxide 85.08 356.8 Citricacid 1,837 — — 1,000 Strontium carbonate 59.18 215.7 monohydrate, 1,954Cobalt carbonate 46.58 92.16 malic acid Iron citrate 18.56 876.7 Ex. 3La_(0.8)Sr_(0.2)Co_(0.2)Fe_(0.8)O₃ Lanthanum carbonate 45.33 847.1Maleic acid 1,910 — — 1,200 Strontium carbonate 59.18 102.3 Cobalthydroxide 62.50 65.17 Hydrated iron 45.58 338.7 hydroxide Ex. 4La_(0.8)Sr_(0.2)Co_(0.8)Fe_(0.2)O₃ Lanthanum hydroxide 73.05 521.5Lactic acid 2,985 — — 750 Strontium carbonate 59.18 101.5 Cobaltcarbonate 46.58 347.0 Hydrated iron 45.58 84.00 hydroxide Ex. 5La_(0.6)Sr_(0.4)Co_(0.8)Fe_(0.2)O₃ Lanthanum hydroxide 73.05 412.1Citric acid 1,973 Diammonium 1,470 800 Strontium carbonate 59.18 213.9monohydrate hydrogencitrate Cobalt carbonate 46.58 365.6 Hydrated iron45.58 88.50 hydroxide Ex. 6 La_(0.594)Sr_(0.396)Co_(0.2)Fe_(0.8)O₃Lanthanum oxide 85.08 355.5 Citric acid 2,907 Ammonium 1,033 800Strontium carbonate 59.18 214.9 monohydrate bicarbonate Cobalt carbonate46.58 92.75 Iron citrate 18.56 882.3 Comp.La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ Lanthanum oxide 85.08 356.8 Citricacid 816 — — 600 Ex. 1 Strontium carbonate 59.18 215.7 monohydrateCobalt carbonate 46.58 92.16 Iron citrate 18.56 876.7 Comp.La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ Lanthanum oxide 85.08 356.8 Citricacid 816 — — 1,200 Ex. 2 Strontium carbonate 59.18 215.7 monohydrateCobalt carbonate 46.58 92.16 Iron citrate 18.56 876.7 Comp.La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O₃ Lanthanum oxide 85.08 356.8 — — — —1,100 Ex. 3 Strontium carbonate 59.18 215.7 Cobalt carbonate 46.58 92.16Iron oxide 69.54 234.0

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a novelLSCF powder having a more highly uniform composition as compared withone obtained by a conventional solid phase method, coprecipitationmethod or slurry method, as an air electrode material powder for a solidoxide fuel cell which is a composite oxide having a perovskite structureand containing lanthanum, strontium, cobalt, iron and oxygen. Bysintering the LSCF powder having a highly uniform composition in theform of a molded product, a LSCF sintered product having an extremelyuniform composition in principle can be obtained since the sinteredproduct takes over this uniform composition.

Further, according to the production process of the present invention, aLSCF powder having a highly uniform composition can be obtained, andthus the present invention is highly industrially useful.

This application is a continuation of PCT Application No.PCT/JP2013/054229, filed on Feb. 20, 2013, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2012-053479 filed on Mar. 9, 2012. The contents of those applicationsare incorporated herein by reference in their entireties.

What is claimed is:
 1. An air electrode material powder for a solidoxide fuel cell, comprising a powder of a composite oxide having aperovskite structure and containing lanthanum, strontium, cobalt, ironand oxygen, wherein the dispersion point determined by the peakintensity [La] of the Lα1 characteristic X-ray of lanthanum and the peakintensity [Sr] of the Lα1 characteristic X-ray of strontium, obtained byEPMA (electron probe microanalyzer) measurement of element distributionof the powder, is present within a range of the formula (1), and thedispersion point determined by the peak intensity [Co] of the Kα1characteristic X-ray of cobalt and the peak intensity [Fe] of the Kα1characteristic X-ray of iron is present within a range of the formula(2):a[La]−150≦[Sr]≦a[La]+150   (1)b[Co]−300≦[Fe]≦b[Co]+300   (2) wherein 0.2≦a≦1.0 and 0.1≦b≦4.0.
 2. Theair electrode material powder for a solid oxide fuel cell according toclaim 1, wherein the composition of the composite oxide is representedby the formula (I):(La_(1-x)Sr_(x))_(a)Co_(y)Fe_(1-y)O₃   (I) wherein 0.1≦x≦0.5, 0.1≦y≦0.9,and 0.95≦a≦1.0.
 3. An air electrode for a solid oxide fuel cell,obtained by sintering the air electrode material powder for a solidoxide fuel cell as defined in claim
 1. 4. A process for producing theair electrode material powder for a solid oxide fuel cell as defined inclaim 1, which comprises forming compounds each containing a metalelement constituting the composite oxide into a solution using anaqueous solution of an organic acid, spray drying the obtained solutionand firing the obtained dry powder.
 5. The process for producing the airelectrode material powder for a solid oxide fuel cell according to claim4, wherein the number of moles of the organic acid used is from 2.3 to10 times the total number of moles of the metal elements of thecompounds each containing a metal element.
 6. The process for producingthe air electrode material powder for a solid oxide fuel cell accordingto claim 4, wherein the organic acid is at least one member selectedfrom the group consisting of maleic acid, lactic acid and malic acid. 7.The process for producing the air electrode material powder for a solidoxide fuel cell according to claim 4, wherein the organic acid is amixture of citric acid with at least one member selected from the groupconsisting of maleic acid, lactic acid and malic acid.
 8. The processfor producing the air electrode material powder for a solid oxide fuelcell according to claim 4, wherein the organic acid is a mixture ofcitric acid with malic acid.
 9. The process for producing the airelectrode material powder for a solid oxide fuel cell according to claim4, wherein the organic acid is citric acid, and when the compounds eachcontaining a metal element constituting the composite oxide are formedinto a solution using the aqueous solution of the organic acid, anammonium compound is further added.
 10. The process for producing theair electrode material powder for a solid oxide fuel cell according toclaim 9, wherein the ammonium compound is at least one member selectedfrom the group consisting of ammonia, ammonium bicarbonate, ammoniumcarbonate and an ammonium citrate.
 11. The process for producing the airelectrode material powder for a solid oxide fuel cell according to claim4, wherein each of the compounds each containing a metal elementconstituting the composite oxide is at least one member selected fromthe group consisting of a carbonate, an oxide, a hydroxide and anorganic acid salt of each metal element.
 12. The process for producingthe air electrode material powder for a solid oxide fuel cell accordingto claim 4, wherein the firing temperature is from 700° C. to 1,300° C.